Additional file 6.

PCA plots of a combined human and mouse gene expression data matrix normalized by
sample. The samples are labeled by (a) species and (b) tissue type. Mouse samples (black) and human samples (red) are well separated on the
axis of component 1. Tissue clusters in the two species are projected to the second
principal component in a similar order: nervous system (blue), muscle/heart (red),
liver (purple) and cell lines (green).